linux-kernel - Re: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX)

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Date:	Fri, 11 Dec 2015 09:43:54 -0800
From:	Andrew Pinski <andrew.pinski@...iumnetworks.com>
To:	Will Deacon <will.deacon@....com>, dave@...olabs.net
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew <Andrew.Pinski@...iumnetworks.com>,
	Davidlohr Bueso <dbueso@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	David Daney <ddaney.cavm@...il.com>
Subject: Re: Commit 81a43adae3b9 (locking/mutex: Use acquire/release
 semantics) causing failures on arm64 (ThunderX)

On Fri, Dec 11, 2015 at 6:18 AM, Davidlohr Bueso wrote:
>
> On Fri, 11 Dec 2015, Will Deacon wrote:
>
>>I think Andrew meant the atomic_xchg_acquire at the start of osq_lock,
>>as opposed to "compare and swap". In which case, it does look like
>>there's a bug here because there is nothing to order the initialisation
>>of the node fields with publishing of the node, whether that's
>>indirectly as a result of setting the tail to the current CPU or
>>directly as a result of the WRITE_ONCE.
>
> Sorry I'm late to the party.
>
> Duh yes this is obviously bogus, and worse I recall triggering a similar tail initialization issue in osq_lock on some experimental work on x86, so this is very much a point of failure. Ack.
>
>>
>>Andrew, David: does making that atomic_xchg_acquire and atomic_xchg fix
>>things for you?

Yes that works for me.  And yes that looks like the correct fix.

>>
>>I don't fully grok what 81a43adae3b9 has to do with any of this, so
>>maybe there's another bug too.
>
> I think this is mainly because mutex_optimistic_spin is where the stack shows the lockup, which really translates to c55a6ffa62.

Yes as mutex_optimistic_spin calls into osq_lock/osq_unlock.  And
81a43adae3b9 changed mutex.c which David thought was where the issue
was located rather than not what mutex_optimistic_spin called.

Thanks,
Andrew Pinski

>
> Thanks,
> Davidlohr
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